PAINT NON-ADHERENCE REAR BUMPER

[RF KILGORE]

On at least two occasions, I have had general body work done on my '99 LEXUS / LS-400 and both times the paint has not adhered properly to the base bumper material.  I have had reputable auto-body and repair shops tell me that the material the base bumper is made of is not allowing the paint coats to adequately adhere and harden.  If the bumper is hit, struck, lightly impacted the paint will crack and begin to peel.  Does anyone know if this is a known issue and if LEXUS has done anything to make necessary repairs / replacements to the bumper?   The car I own is in otherwise excellent condition except for the cracking and peeling of the paint on the rear bumper.  Does anyone have suggestions of how to remedy this problem or replace this bumper with a different type of material manufactured to allow the paint to properly adhere to the surface.